The long-term goal of this project is to understand how solutes and water cross epithelia specialized for fluid transport, such as intestine, stomach, kidney urinary bladder, and gallbladder. We propose to develop new electrical methods, based on impedance analysis, for simultaneously determining three sets of properties of epithelia: resistances of cell junctions and of apical and basolateral cell membranes; real areas (measured as capacitances) of apical and basolateral membranes; and reflections of ultrastructural geometry, especially membrane folding, in two electrical phenomena termed series resistances and distributed effects. Impedance analysis potentially offers advantages for determining these properties and hence unravelling functional organization in epithelia. Methods for measuring impedance in tight epithelia by sequential transepithelial application of single alternating-current frequencies will be developed on rabbit urinary bladder. The additional technique of measuring alternating-current voltage-divider ratios is required to extend this approach to leaky epithelia, and will be developed on Necturus gallbladder. Applied noise signals will be used as a technique for studying fast processes and for measuring impedance in small cells. These new techniques will then be applied to studying the functional organization of gastric mucosa, an important and interesting but complicated epithelium whose organization and transport processes have been difficult to study with existing methods.